The only thing I noticed, when I put my 170 T-stat in, was a new, annoying, REALLY LONG warm up time. The lower temp 170 T-stat opens at a lower temperature, so the coolant starts flowing between the block and radiator at a lower temperature, which now means that the engine takes longer to reach full warm up, since you have to heat ALL the coolant now, and that has to occur while while its flowing through a cooling device, namely the radiator, as opposed to just warming the coolant in the block before full circulation starts. THis stretches out the warm up time dramatically. Yes, i'm a nut who likes all the internal engine parts to be in a relatively stablized state of thermal expansion, before I put a load on the engine.
Under heavy load conditions, (when you got your foot in it, stop and go, hot days, tracking in sunny climates), the T stat is generally OPEN, regardless, and there is no difference between the two.
A 190 T stat maintains the engine at a higher temperature, during light load or cold conditions. In So Cal, Id bet that usually, its open, and the coolant circulates through the radiator during most of the driving cycle. If and when the coolant temperature drops to about 190, it closes. Thats the way to think about it, the rating is the temperature at which it closes, IF its in a cooling phase, IF the conditions warrant. The coolant is then restricted to the block only. This temporarily reduces the effectiveness of the cooling system, maintaining a higher average engine temperature. As the engine heats up, it opens again, allowing coolant once again to flow through the radiator. A higher temperature T stat maintains higher engine temperatures this way. It wont allow the engine to cool too much, under light loads or cold conditions. I think thats good for low emissions, and figures in when they design for smog certification.
A 170 T stat allows the engine to swing down to a lower temperature, under light load or cold conditions, since it doesnt close the flow to the radiator until the coolant has reached the lower temperature of 170. Once the engine begins to heat up, it opens again, allowing cololant once again to flow through the radiator. NO extra capacity of the cooling system is granted by a 170 T stat, strictly speaking. It only lowers the temp at which the coolant is restricted to the block, and allows for wider temperature swings during the right conditions, as Marv mentioned. The radiators' ability to dissipate BTUs into the atmosphere is completely unaffected. A 170 degree T stat will not help, if you are overheating in a steady state of load and ambient temperature. It does not add any cooling ability to your car, per se. But thats not the whole story!
Here's where a 170 T stat can make a difference, under the right conditions. Consider the situation, where you are alternating between light load, and heavy load, (you pass some cars with vengence, then settle down for a bit, then you're back on it again). During a cooling phase, (if conditions allow such) the lower temp T stat would allow the coolant to reach that lower rated temperature of 170 before closing, allowing the engine to cool more, giving it more room to absorb that much more heat on its way back up, during a subsequent heating phase. You gain 20 degrees of heat "soakability", which is then dissipated by the radiator in the normal way. As long as there is a cooling phase following, there will be this range of ability to dissipate heat. Yes, open is still open, the coolant is still the coolant, the radiator doesnt get any bigger, nothing changes in that regard, 170 vs 190,.... but ..... you gain those 20 degrees of "ramp up room". The 190 allows say, 60 degrees of heat soak until 250 (arbitrarily chosen freak out temp) where the 170, under intermittant load conditions only, allows 80 degrees of heat soakability before 250 is reached, when its time to be concerned. Before someone feels compelled to state the obvious, no, this makes no difference under continuous heavy load conditions, like track use. At that time, either a 170 or a 190 thermostat will be open, and the cooling system will be extracting its maximum amount of BTUs from the engine by viture of its design, regardless of T stat rating, because as Tom said, "open is open".
Intermittant load use is the only time I could see any advantage to a 170 T stat, here in generally hot So Cal. Unless you drive at night? But then you dont need the extra heat soak margin, because of the lower ambient temps.
Here in sunny So Cal, I'd be surprised if the T-stat ever closes at all during regular driving, once it has opened. So, during my particular driving cycle, if it opens only once during the inital wamup, but then never closes again, whats the point? Might as well not have a T stat at all, just let the coolant circulate continuously, under all conditions. Its not like the engine is going to freeze to death. If anything, its always hot. Just open the hood after a long drive, feel that huge wave of heat, and you get the picture. Its not like it regulates the temperature to the 170 degree value, thats only the temp that it opens up at during a cooling phase, under light loads or cold temperatures. A lower temp T-stat may not make any difference here, in our fairly hot driving conditions, but I havent done any study of it yet. THis is the first time I have considered the subject, just shooting from the hip.
During a cooling phase, if conditions allow, my 170 T stat allows the engine to cool to that lower temperature, before it closes, and restricts the coolant flow to within the block only. The effect would exhibit itself by the temp gauge needle swinging way down towards the left side of the dial, before it closes, and then the temp would rise again, until the cooling system has bled off the heat again. Since I almost never cruise under low load conditions, or in lower ambient temperatures, I dont think this scenario applies in my case. But Im not entirely sure, since I dont generally watch my gauge intently, unless I'm on the track. If its pointing something like straight up, thats ok with me! (Im referring the gauge needle) I'd be surprised if T stats ever close in So Cal. But if they do, then a 170 would allow more ramp up room. It deserves further investigation.
Any other thoughts on the issue?
My bad !! I knew that you couldn't be talking about the housing that the thermostat is in. Maybe we're talking about drilling the hole in the same place; I just didn't know what to call it.
